1. Field of the Invention
The present invention relates to a fuel injection device applied to engines such as diesel engines or direct injection type gasoline engines.
2. Description of the Prior Art
Conventional fuel injection devices that control fuel injection into combustion chambers of engines such as diesel engines include those disclosed in Japanese Patent Laid-Open Nos. 965/1991 and 171266/1992. These fuel injection devices have a needle valve that opens or closes nozzle holes formed at the front end of an injection nozzle and control the fuel injection by the balance between a force produced by a fuel pressure acting on the needle valve on the nozzle front side in a direction that opens the nozzle holes and a force produced by a fuel pressure in a balance chamber acting in a direction that closes the needle valve.
The fuel injection device of a type disclosed in the Japanese Patent Laid-Open No. 171266/1992 uses a three-way valve to open and close an exhaust passage that releases the fuel pressure in the balance chamber. As shown in FIG. 8 and 9, the three-way valve 54 of the fuel injection device electromagnetically switches, according to a control signal from a control unit, between a balance chamber passage 51 communicating with the balance chamber, a supply passage 52 connecting to a fuel supply pump through a common rail, and an exhaust passage 53 leading to a reservoir and thereby controls the start and stop of fuel injection.
As shown in FIG. 8, when the three-way valve 54 connects the supply passage 52 to the balance chamber passage 51 and closes the exhaust passage 53, the balance chamber recovers a high fuel pressure causing the needle valve to move down to stop fuel injection.
As shown in FIG. 9, when the three-way valve 54 operates to connect the balance chamber to the exhaust passage 53 through the balance chamber passage 51 and at the same time closes the supply passage 52, the high pressure fuel in the balance chamber leaks into the exhaust passage 53 through the balance chamber passage 51 and the balance chamber pressure decreases allowing the needle valve, whose end is exposed to the balance chamber, to lift to inject fuel through the open nozzle holes. The closure of the supply passage 52 blocks the high fuel pressure from entering into the balance chamber.
A fuel injection device of a type disclosed in Japanese Patent Laid-Open No. 965/1991 on the other hand uses a two-way valve for opening and closing an exhaust passage 65 that releases the fuel pressure from the balance chamber 62, as shown in FIG. 10. The balance chamber 62 is formed in a fuel injection device body 61 above a control piston 60 connected to the needle valve. The balance chamber 62 communicates with a supply passage 63 through which fuel is supplied from a fuel source and in which a throttle 64 is formed. The exhaust passage 65 for discharging fuel from the balance chamber 62 comprises a fuel passage 66 and an orifice 67. The orifice 67 is opened and closed by a solenoid valve 68 driven by a control signal from the control unit.
When the orifice 67 is opened by the solenoid valve 68, the fuel is released through the exhaust passage 65. Because the supply of fuel from the supply passage 63 is limited by the throttle 64, the fuel pressure in the balance chamber 62 decreases, causing the control piston 60 and therefore the needle valve to lift to inject fuel. When the orifice 67 is closed by the solenoid valve 68, the discharge of fuel from the exhaust passage 65 is stopped. As the fuel is supplied through the supply passage 63 and throttle 64, the fuel pressure in the balance chamber 62 recovers pushing down the control piston 60 and therefore the needle valve to stop fuel injection.
Another example of the fuel injection device, disclosed in Japanese Patent Laid-Open No. 244864/1986, uses a ball valve 73 having a stem passing through an exhaust passage 71 to open and close the exhaust passage 71 that releases the fuel pressure from a balance chamber 70, as shown in FIG. 11. The ball valve 73 works as a three-way valve that opens and closes fuel passages 76, 77 and the exhaust passage 71 by opening and closing ports 78, 79. FIG. 11 shows a state in which an actuator 74 of the solenoid valve is operated to push out the valve stem 72 to close the fuel passage 76 that branches from a fuel passage 75 communicating with a high pressure fuel source. When the ball valve 73 closes the port 79 of the fuel passage 76, the balance chamber 70 to which the end of the needle valve 80 is exposed communicates with the exhaust passage 71. The fuel pressure in the balance chamber 70 is then released into the exhaust passage 71 and the fuel pressure acting on a pressure receiving surface 82 of the needle valve 80 lifts the needle valve 80, injecting fuel from nozzle holes 81. When the actuator 74 is deenergized and the ball valve 73 closes the port 78 of the exhaust passage 71, the pressure of the fuel passage 76 is transmitted through the fuel passage 77 to the balance chamber 70, pushing down the needle valve 80 to stop fuel injection from the nozzle holes 81.
The fuel injection device of a type disclosed in Japanease Patent Laid-Open No. 171266/1992 such as shown in FIG. 8 and 9, however, has the drawback that although fuel seems to be not wasted because the three-way valve that opens and closes the exhaust passage closes the supply passage communicating with the balance chamber, the high pressure fuel actually leaks through a sliding clearance in the three-way valve, resulting in lower fuel efficiency than in the two-way valve. When, as shown in FIG. 8, the valve member 56 of the solenoid valve lowers relative to the valve shaft 55 to allow communication between the balance chamber passage 51 and the supply passage 52 through an open seal portion 58 and close the exhaust passage 53, which leads to the reservoir, with respect to the balance chamber passage 51 and the supply passage 52 by a seal portion 57, it is found that the high fuel pressure from the supply passage 52 leaks through a sliding clearance 59 between the solenoid valve body 56 and the fuel injection device body. It is also found that when, as shown in FIG. 9, the solenoid valve disc 56 moves up allowing communication between the balance chamber passage 51 and the exhaust passage 53 through the open seal portion 57 and closing the supply passage 52 by the valve shaft 55 at the seal portion 58, the high pressure fuel from the supply passage 52 leaks through the sliding clearance 59 between the solenoid valve member 56 and the fuel injection device body.
In the fuel injection device of a type disclosed in Japanease Patent Laid-Open No. 965/1991 shown in FIG. 10, the opening and closing of the exhaust passage 65 connected to the balance chamber 62 is controlled by the solenoid valve 68 as a two-way valve, which presses against the end of the exhaust passage 65 from outside to keep the fuel in the balance chamber 62. So, the solenoid valve 68 is always acted upon by a fuel pressure in a valve opening direction. Under this condition when it is attempted to inject the fuel at high pressure, the solenoid valve 68 is opened by the high fuel pressure which then leaks through the solenoid valve even while the fuel injection is not performed. The fuel leakage constitutes a wasted work of the fuel injection pump, degrading the mileage.
To prevent the solenoid valve from being opened by the fuel pressure requires increasing the force of a return spring of the solenoid valve. This unavoidably increases the size of the actuator (solenoid) that opens the solenoid valve against the force of the spring, which in turn poses such problems as increased manufacturing cost, increased power supplied to the solenoid of the actuator to drive the solenoid valve against the strong spring force, and increased size of the fuel injection device itself. Further, because the valve seat portion of the solenoid valve is formed around the port of the exhaust passage, the area of the valve seat portion is small, so that when the valve is closed a high surface pressure is produced at the valve seat portion by the impact of the valve disc of the solenoid valve caused by the strong spring force. The valve seat portion is therefore easily worn, which in turn causes fuel leakage from the worn seat increasing the overall leakage.
In the fuel injection device of a type disclosed in Japanease Patent Laid-Open No. 244864/1986 shown in FIG. 11, because the valve disc that opens and closes the exhaust passage is a ball valve 73, the seal portion where the ball valve 73 closes the exhaust passage 71 has a line contact, which means that the pressure acting on the seal portion is very high. This unavoidably wears the seal portion and causes fuel leakage through the worn part. Further, a turbulent flow of fuel generated around the ball valve 73 when it is activated causes the ball valve to vibrate, making the lift adjustment of the valve member and the fuel injection rate control impossible.
In the conventional fuel injection devices for engines, as described above, neither the three-way valve nor the two-way valve, both used to open and close the exhaust passage that releases the fuel pressure in the balance chamber, can prevent fuel leakage through these valves. As a result the fuel pump is burdened with wasted work, deteriorating the mileage.